Fibrillar Adhesives in Nature
Nature provides endless inspiration for solutions to engineering challenges. Particularly at the small (sub-millimeter) scale, millions of years of evolution has resulted in fascinating structures with unique, sometimes non-intuitive properties. In the case of small agile climbing animals, fibrillar foot-pads as a solution for gripping surfaces has evolved many times. Similar structures are present in animals of different phyla, including arthropods (spiders, insects), and chordates (lizards), suggesting independent evolution. There is also evidence that these structures evolved independently within different types of lizards (Geckos, Anoles, and Skinks), with slightly different resulting structures [D. Irschick, A. Herrel, and B. Vanhooydonck, “Whole-organism studies of adhesion in pad-bearing lizards: creative evolutionary solutions to functional problems,” Journal of Comparative Physiology A: Neuroethology, Sensory, Neural, and Behavioral Physiology, vol. 192, no. 11, pp. 1169-1177, 2006].
There exist a wide variety of fibrillar adhesives across the wide variety of animals, which utilize these structures. Some insects have fibrillar foot pads which secrete oily fluids which aid in adhesion, while others have completely dry structures. Adhesive pads which do not utilize secretions are called “dry adhesives,” as they leave no residue on the surfaces to which they adhere. Dry adhesives exhibit many unique adhesive properties. They act similar to a pressure sensitive adhesive such as tape, but are highly repeatable with long lifetimes, do not require cleaning, and, often in combination with small claws, adhere to surfaces which are anywhere from atomically smooth silicon to extremely textured rock. Furthermore, they exhibit directional properties, adhering in one direction, and easily releasing from the surface when loathed in another. Adhesion pressures as high as 200 kPa have been demonstrated for gecko subdigital toepads and single fiber (seta) measurements exhibited adhesion pressures greater than 500 kPa (50 N/cm2) [K. Autumn, “Biological Adhesives,” Springer Berlin Heidelberg, 2006]. Using advanced fibrillar adhesives, several gecko species are capable of carrying up to 250% of their own body weight up a smooth vertical surface. Dry fibrillar adhesives are also quite power efficient. They can be attached and detached from surfaces with very low forces by means of special loading and peeling motions. Once adhered to a surface, they require no power to maintain contact, and resist detachment for long periods of time.
Interestingly, and against intuition, the material that makes lip these high performance adhesive footpads is not sticky at all. The fibers are made from a β-keratin, much like bird claws and feathers. It is the small size-scale and geometrical structure, which allows this material to act as a powerful and versatile attachment mechanism.
Mechanics of Fibrillar Adhesion
The hair-like structures of gecko footpads have fascinated scientists for well over a century, with various hypotheses about the mode of attachment. In 1884, Simmermacher proposed the hypothesis that gecko lizards might adhere to surfaces using micro-suction cups [G. Simmermacher, “Untersuchungen ber haftapparate an tarsalgliedem von insekten,” Zeitschr. Wiss. Zool, vol. 40, pp. 481-556, 1884]. Fifty years later, Dellit carried out experiments in a vacuum winch demonstrated that suction is not the dominant attachment mechanism in geckos [W. D. Dellit, “Zur anatomie and physiologie der geckozehe,” Jena. Z. Naturw, vol. 68, pp. 613-656, 1934]. Similarly, electrostatic adhesion and micro-interlocking were ruled out. It was not until the advent of the Scanning Electron Microscope that scientists were able to investigate the true structure of these microscopic features. What they observed is a forest of microscale fibers, each branching into finer and finer hairs, ending in spatula-like tips. It is this structure that turns the stiff keratin into a capable adhesive.
Conventional pressure sensitive adhesives such as adhesive tapes, gels, and soft elastomers function by deforming into the shape of the contacting surface when pressed into contact. Materials with very low Young's modulus (stiffness) conform to surfaces to create large contact areas and do not store enough elastic energy to induce separation from the surface after the loading is removed. However, due to their low modulus, these materials tend to pick up contaminants from the surface, and are typically not re-usable.
Stiffer materials do not easily conform to surface roughness, and if deformed into intimate contact through high loading, store enough elastic energy to return to their original shape, peeling away from the surface in the process of relaxation. Bulk stiff materials generally do not exhibit tackiness or adhesion due to this self-peeling behavior.
The structures found in the attachment pads of the animals described above consist of arrays of thousands or millions of hair-like fibers, which stanch vertically or at an angle from the pad surface. Each fiber acts independently and generally has a specialized tip structure. The hairs in these fibrillar adhesives conform to the roughness of the climbing surface to increase the real contact area much like the deformation of soft adhesive tape, resulting in high adhesion by surface forces [K. Autumn et al., “Adhesive force of a single gecko foot-hair,” Nature, vol. 405, pp. 681-685, 2000]. This adhesion, called dry adhesion, is argued to arise from molecular surface forces such as van der Waals forces [K. Autumn et al., “Evidence for van der Waals adhesion in gecko setae,” Proceedings of the National Academy of Sciences USA, vol. 99, pp. 12252-12256, 2002], possibly in combination with capillary forces [G. Huber et al., “Evidence for capillarity contributions to gecko adhesion from single spatula nanomechanical measurements,” Proceedings of the National Academy of Sciences USA, vol. 102, pp. 16293-16296, 2005]. Although the total potential contact area of a surface broken up into fibers is less than the area of a flat surface because of the gaps between the fibers, the ability for each fiber to bend and conform to the surface roughness allows thousands, millions or billions of fibers no make small individual contacts, which add up to a large surface area. In comparison, a flat surface only makes contact with the asperities of a surface, and since the deformations of bulk material are typically small, the total contact area is much less than in the fibrillar case. An illustration of this comparison can be seen in FIGS. 1a and 1b, which illustrate the contact area of a flat stiff material 2 against a rough surface 4 (FIG. 1a) can be less than the contact area of a fibrillar adhesive 6 against the same surface 4 (FIG. 1b) despite the area lost between the fibers.
Because of the high aspect ratio (height to diameter) of the fibers in FIG. 1b, the fibrillar surface's effective modulus is low despite the material modulus typically being quite high. The keratinous materials found in geckos' fibers are estimated to have a Young's modulus of approximately 1-2.5 GPa. However, due to their hairy structure, the effective modulus is closer to 100 kPa, much like a soft tacky elastomer.
Animals with very low mass, such as insects, generally have a simply micro-fiber structure with specialized tips. In large lizards such as the Tokay gecko the fibers take on a complicated branched structure with microscale (4-5 μm) diameter base fibers which branch down to sub-micron (200 nm) diameter terminal fibers. At the end of these terminal fibers are specialized tips.
The most advanced fibrillar dry adhesives are found in the heaviest animals such as the Tokay and New Caledonia Giant Gecko gecko which can weigh up to 300 grams. Gecko toes have been shown to adhere with high interfacial shear strength to smooth surfaces (88-200 kPa). These animals have adhesive pads with many levels of compliance including their toes, foot tissue, lamellae, and fibers. This multi-level hierarchy allows the adhesives pads to conform to surface roughness with various frequency and wavelength scales. The fibers are angled with respect to the animals' toes, and the branched tips are also oriented with respect to the base of the fiber. The result is that the gecko pad exhibits a high level of directional dependence, high adhesion while dragging the toe inwards, and no adhesion in the opposite direction. This directionality is sometimes referred to as frictional anisotropy or, more appropriately, directional adhesion.
Studies of gecko footpads have revealed that due to their asymmetric angled structure, they are non-adhesive in their resting state, and a dragging motion is required to induce adhesive behavior [K. Autumn et al., “Frictional adhesion: a new angle on gecko attachment,” Journal of Experimental Biology, vol. 209, pp. 3569-3579, 2006]. Reversing the direction of this dragging motion removes the fibers from the surface with very little force.
Motivation for Fabrication of Dry Adhesives
The dry fibrillar adhesive structures found in nature exhibit properties, which may be highly desirable in synthetic materials. The mechanics which gives rise to the adhesion in these structures does not rely on liquids or pressure differentials, therefore fibrillar dry adhesives are uniquely suited for a variety of uses. Since dry adhesives leave no residue and can grip over large areas, they could be used as grippers for delicate parts for transfer and assembly of anything from computer chips in a clean-room to very large porous carbon-fiber panels for vehicle construction.
If manufactured inexpensively, synthetic dry adhesives could also find uses in daily life as a general adhesive tape for hanging items, fastening clothing, or as a grip enhancement in athletic activities such as gloves, shoes, and grips.
Man-made dry adhesives might be used for temporary attachment of structures during assembly, or allow astronauts to grip the smooth outer surfaces of spacecraft during extravehicular missions.
Since biological dry adhesives allow animals to climb on smooth surfaces, synthetic dry adhesives should enable robots to do the same. Robots with dry adhesive grippers may be used for inspection and repair of spacecraft hulls, or terrestrial structures. Since the adhesives require no power to remain attached, climbing robots could perch for days, weeks, or months with very little power usage. Also, due to the power efficient attachment and detachment, robots might move as easily up a wall as they currently traverse the ground. Similarly, one day, gloves covered in synthetic dry adhesives might allow humans to easily scale smooth vertical surfaces.
There are potential applications for fibrillar adhesives in the field of medicine as well. Safe, non-destructive temporary tissue adhesives could assist in surgical procedures. Capsule endoscopes might use fibrillar adhesives to anchor to intestine walls without damaging the tissue in order to closely examine or biopsy an area of interest. Fibrillar adhesives may also be designed for attachment to skin as an alternative to conventional adhesive bandages and patches.